Add a layout debug module. This outputs a trace of the layout process to a JSON
file which can be viewed in an external tool. It provides a timelapse view of
how the flow tree and fragments changed during the layout process, which makes
it easier to debug layout bugs.
Change scope macro to be a no-op in release builds.
Avoid trying to trace a null JSVal.

JSVal::trace_kind() asserts that it is a markable type; null is a gcthing that
is not markable.
Merge pull request #3240 from Ms2ger/mark-null

Avoid trying to trace a null JSVal; r=jdm
Merge pull request #3206 from glennw/layout-trace-enc

Add an initial pass at a layout debug module.
Fix WPT when /usr/bin/python is python3

Use the Python that was detected during configure.
Merge pull request #3246 from SimonSapin/wpt-python2

Fix WPT when /usr/bin/python is python3